Sen. Robert Menendez finds himself under FBI investigation amid allegations of influence peddling.

Now the Garden State’s other senator, 89-year-old Frank Lautenberg, has called it quits. His announcement comes in the wake of a primary challenge from Newark Mayor Cory Booker.

It also comes just 48 hours after The New York Times reported that Lautenberg was embracing his job “with new vigor” and “brightens at the prospect of tussling with a man half his age.”

Oh, well.

In once sense, Lautenberg’s retirement is a historic milestone: He is the last WWII veteran to serve in the Senate.

And he managed to set something of a record for New Jersey by avoiding the ethical problems that ended the careers of his Democratic predecessors.

These include Sen. Harrison Williams, convicted in the ’80s Abscam corruption sting. They also include Sen. Bob Torricelli, who faced influence-peddling allegations similar to those now swirling around Menendez — and whose last-minute withdrawal from his re-election in 2002 paved the way for Lautenberg’s return to the Senate.

Now, after five terms of faithful adherence to his party, he’s stepping aside. “Voluntarily,” of course.